predictions and wind tunnel - cfd4aircraft · 2007. 7. 12. · pitchup l pitchup lift is also...
TRANSCRIPT
Comparison of Numerical Predictions and Wind Tunnel
Results for a Pitching Uninhabited Combat Air Vehicle
Russell M. Cummings, Scott A. Morton, and Stefan G. Siegel
Department of Aeronautics United States Air Force Academy
USAF Academy, CO
Outline
l Introduction l Dynamic Stall/Lift l UCAV Configuration l Experimental Results l Numerical Method l Static Results l Pitching Results l Conclusions
Introduction
l UCAV’s are playing an important role in current military tactics
l Predator and Global Hawk are becoming essential elements of current operations
l X45A represents future configurations
Introduction
l Some issues to explore in order to take advantage of a UCAV’s uninhabited state: – High g maneuvering – Compact configurations – Novel control actuation – Morphing wings – MEMSbased control systems – Semiautonomous flight – Increased use of composites – Novel propulsion systems – Dynamic stall/lift
Dynamic Stall/Lift
l Utilizes rapid pitchup and hysteresis to produce increased lift
l A great deal of work has been done on airfoils and simple wings
l Very little work has been done on UCAVs
From http://hodgson.pi.tuberlin.de/~schatz/PIZIALI/osc.html
a—separation begins b—leadingedge vortex forms c—full stall d—reattachment and return to static state
UCAV Configuration
l Straight, swept leading edge with 50 o sweep
l Aspect ratio of 3.1 l Round leading edges l Blended wing & body l Top/front engine inlet l B2like wing planform l Low observable shaping
Boeing 1301 UCAV
33.27
62.43 276.15 46.19 ft
78.78
33.27
32.58 ft
MAC 241.93 BL 92.4
MAC/4 FS 170.63
24.19 50°
30°
30°
30°
27.38 sq ft
21.02 sq ft
Experimental Results
l 1:46.2 scale model l Academy 3 ft × 3 ft open return lowspeed wind tunnel
l Less than 0.05% freestream turbulence levels at all speeds
l Freestream velocity of 20 m/s (65.4 ft/s)
l Chordbased Reynolds number of 1.42 × 10 5
Static Testing
l Linear lift characteristics up to 10 o to 12 o
l Stall occurring at about 20 o
l Lift reestablished up to 32 o , where an abrupt loss of lift takes place
l Effect of leadingedge vortices and vortex breakdown? Angle of Attack, α
Force Coe
fficien
t
0 10 20 30 40 50 60 70 0
0.2
0.4
0.6
0.8
1
1.2
1.4
Lift Coefficient Drag Coefficient
Dynamic Testing
l The configuration was pitched at 0.5, 1.0, and 2.0 Hz (k = 0.01, 0.02, and 0.04)
l Center of rotation at the nose, 35% MAC, and the tail
l The pitch cycles were completed for three ranges of angle of attack: – 0 o < α < 20 o
– 16 o < α < 35 o
– 25 o < α < 45 o
)) cos( 1 ( ) ( t m t ω α α − + = o
Pitching About 35% MAC @ 2 Hz
l Dynamic lift is greater than static lift during pitchup
l Pitchup lift is also greater post stall
l Dynamic lift is less than static lift during pitchdown
l Little impact on drag Angle of Attack, α (deg)
Force Coefficien
t
0 5 10 15 20 25 30 35 40 45 0.5
0.25
0
0.25
0.5
0.75
1
1.25
1.5
α(t) = 16 o to 35 o α(t) = 25 o to 45 o
α(t) = 0 o to 20 o
Static Drag Static Lift
Pitching About Nose @ 2 Hz
l Similar results to pitching about 35% MAC
l Slightly less effective at producing lift during pitchup
l Essentially identical results in poststall region
Angle of Attack, α (deg)
Force Coefficien
t
0 5 10 15 20 25 30 35 40 45 0.5
0.25
0
0.25
0.5
0.75
1
1.25
1.5
α(t) = 25 o to 45 o
α(t) = 0 o to 20 o
α(t) = 16 o to 35 o
Static Drag Static Lift
Pitching About Tail @ 2 Hz
l Drastically different than previous results
l Much more lift at higher angles of attack in pitching cycle
l Reduced lift at lower angles
l Significant impact on drag Angle of Attack, α (deg)
Force Coefficien
t
0 5 10 15 20 25 30 35 40 45 0.5
0.25
0
0.25
0.5
0.75
1
1.25
1.5
α(t) = 25 o to 40 o
α(t) = 0 o to 20 o
α(t) = 16 o to 35 o
Static Drag Static Lift
Numerical Method
l Cobalt NavierStokes solver – Unstructured mesh – Finite volume formulation – Implicit – Parallelized – Secondorder spatial accuracy – Secondorder time accuracy with Newton subiterations
l Run on Academy 64 processor Beowulf cluster, Origin 2000, and USAF HPC computers
l Laminar flow with freestream conditions set to match Reynolds number of wind tunnel experiment
Mesh and Boundary Conditions
l Three unstructured meshes: – Coarse (1.3 million cells) – Medium (2 million cells) – Fine (4 million cells)
l Halfplane model l Noslip on surface l Symmetry plane l Freestream inflow l Inlet covered to match model l No sting modeled
Mesh Convergence
l Steady results from all three meshes yield identical forces
l High angle of attack flowfields will be unsteady
l Use 2 million cell mesh for following calculations
l Detailed time step study to follow for unsteady flow
Number of Iterations
NormalFo
rce
250 500 750 1000 2
1
0
1
2
3
4
Fine Mesh (4 million cells) Medium Mesh (2 million cells) Coarse Mesh (1.3 million cells)
o 20 = α
SteadyState Static Results
l Good results in linear angle of attack range
l Qualitatively similar results in poststall region
l Lift and drag are significantly over predicted in poststall region Angle of Attack, α
Force Coe
fficien
t
0 10 20 30 40 50 60 70 0
0.2
0.4
0.6
0.8
1
1.2
1.4
Exp. Lift Coefficient Exp. Drag Coefficient CFD Lift Coefficient CFD Drag Coefficient
SteadyState Static Results
l Wide shallow vortices l Vortex breakdown fairly far back on configuration
l Vortical structures behind breakdown maintain lift on aft of vehicle
l Rounded leading edge creates weaker vortices that breakdown sooner
o 20 = α
TimeAccurate Static Results
l Flowfields in poststall region are unsteady
l Timeaccurate results match experiment much more closely
l Fairly good modeling of flowfield, including drag, up to α=45 o
l Differences in lift from α=20 o to α=30 o (sting, surface roughness, transition ?)
Angle of Attack, α (deg)
Force Coe
fficien
t
0 10 20 30 40 50 60 70 0
0.2
0.4
0.6
0.8
1
1.2
1.4
Exp. Lift Coefficient Exp. Drag Coefficient CFD Lift Coefficient CFD Drag Coefficient
TimeAccurate Static Results
o5 = α
o 15 = α
o 10 = α
o 20 = α
TimeAccurate Static Results
o 25 = α
o 35 = α
o 30 = α
o 40 = α
Dynamic Pitching Results
l Pitchup doesn’t capture full lift increase at low α
l Overprediction of lift also seen in pitchup case
l Drag is fairly well modeled during pitchup
l More cycles required Angle of Attack, α (deg)
Force Coe
fficien
t
0 5 10 15 20 25 0.25
0
0.25
0.5
0.75
1
1.25
Exp. Lift Coefficient Up Exp. Drag Coefficient Up Exp. Lift Coefficient Down Exp. Drag Coefficient Down Exp. Static Lift Coefficient Exp. Static Drag Coefficient CFD Lift Coefficient CFD Drag Coefficient
Δt*=0.0075, nsub=5
Dynamic Pitching Results
Static Pressure Pitching Pressure
o 15 = α
Conclusions
l A generic UCAV configuration has been wind tunnel tested both statically and pitching
l The configuration generates increased dynamic lift during pitchup maneuver
l Numerical simulation helps to understand causes of wind tunnel results – Stronger leadingedge vortex during pitchup – Leadingedge vortex persists to very high angles of attack – Vortex breakdown causes the nonlinearities in lift
l Collaboration between experimentalists and computationalists leads to greater understanding of aerodynamics
Questions?